Although several nitrogen oxides are known which are relatively stable at ambient conditions, it is generally recognized that two of these, viz. nitric oxide (NO) and nitrogen dioxide (NO.sub.2), are the principle contributors to smog and other undesirable environmental effects when they are discharged into the atmosphere. These effects will not be discussed further here since they are well recognized and have led various government authorities to restrict industrial emissions in an attempt to limit the level of the oxides in the atmosphere. Nitric oxide and nitrogen dioxide, under appropriate conditions, are interconvertible according to the equation EQU 2NO+1/2O.sub.2 =2NO.sub.2.
For purposes of the present invention, NO.sub.x will be used herein to represent nitric oxide, nitrogen dioxide, and mixtures thereof.
Formation on man-made nitrogen oxides from the elements occurs in the high temperature zones of combustion processes. The internal combustion engine, and coal or air-fired furnaces, boilers and incinerators, all contribute to NO.sub.x emissions. In general, fuel-rich combustion mixtures produce exhaust gases with lower contents of NO.sub.x than do lean mixtures. Although the concentrations of NO.sub.x in the exhaust gases produced by combustion usually are low, the aggregate amounts discharged in industrial and/or highly populated areas is adequate to cause problems. Other industrial sources of pollution also exist. These are associated with the manufacture of nitric acid, with nitration of organic chemicals, and with other chemical operations such as the reprocessing of spent nuclear fuel rods by dissolution in nitric acid to recover uranyl nitrate followed by calcination to convert the nitrate to uranium oxide. In these instances the waste gases may contain relatively high levels of NO.sub.x, such as from 0.1% up to 2% to 3%.
The so-called "stable" nitrogen oxides have in common the somewhat peculiar property that although they are thermodynamically very unstable with respect to decomposition into elemental oxygen and nitrogen, no simple, economical method has been described for inducing this decomposition. It has been discovered, however, that adding a reductant such as ammonia to the exhaust gas can, under appropriate reaction conditions, convert NO.sub.x to elemental nitrogen and steam.
U.S. Pat. No. 3,900,554 to Lyon describes a homogeneous gas phase reaction to remove NO.sub.x from combustion effluents by adding 0.4 to 10 mols (preferably 0.5 to 1.5 mols) of ammonia followed by heating to 1600.degree. C. to 2000.degree. C. The NO.sub.x content is lowered as a result of its being reduced to nitrogen by reaction with ammonia. The method is reported to work best if hydrocarbon is also added to the mixture.
U.S. Pat. No. 4,220,632 to Pence et al. discloses a process for reducing noxious nitrogen oxides from a fossil-fuel-fired power generation plant, or from other industrial plant off-gas stream, to elemental nitrogen and/or innocuous nitrogen oxides employing ammonia as reductant and, as catalyst, the hydrogen or sodium form of a zeolite having pore openings of about 3 to 10 Angstroms.